(b)
BITE monitors and tests the following: 1) Power supply 2) Laser gyros and accelerometers 3) Pitch rate output 4) Discrete Input/Output 5) Sensor electronics 6) CPU 7) Input receiver and output transmitters 8) Alignment, air data input 9) Derived latitude compared to entered latitude 10) Keyed in longitude compared to stored longitude 11) Memory operation 12) Instruction execution and data parity.
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A
737-300/400/500MAINTENANCE MANUAL
(c)
The watch dog timer (WDT) monitors and detects failures that prevent the CPU from executing its program. The CPU is programmed to reset the WDT every 20 ms (50 Hz rate). If the CPU is unable to reset the WDT because of a computer failure, hangup, or memory failure, the WDT will time out. This will set the fault ball and light the FAULT annunciator. In addition, the analog pitch rate will be invalidated, and a fault word will be written into BITE memory.
(d)
Non-volatile BITE memory states the latest calculated longitude and accumulated BITE status record of the last two power-on periods. It also stores a summary of IRS discrete message words for the last six power on periods.
(e)
When BITE detects a failure, the analog pitch rate invalid discrete is set. The fault annunciator on the MSU is also lit and the fault ball is set.
(2) Interface Test
(a) Pressing the test switch on the IRU or setting the data display selector switch on the ISDU to the TEST position, activates the manual interface test mode. This test mode is inhibited in the NAV mode when the ground speed exceeds 20 kts and in the ATT mode. The manual interface test causes the following to occur: 1) From 0 to 2 seconds, all MSU annunciator discretes are
activated, causing all four MSU annunciator to come on. Also, all ISDU display segments come on during this two second period.
2) From 2 to 10 seconds, all failure discretes and digital failure words are set to FAIL. The more visible results are that the ADI, HSI, and RDDMI fault flags come into view.
3) After 10 seconds, each output of IRS is set to a test value until the test switch is released.
4) Figure 23 shows test values of the IRU output parameters during self-test. Those parameters identified by flagnote 1 can be checked on the ISDU and/or the pilots' instruments.
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A 737-300/400/500MAINTENANCE MANUAL
LABEL/PARAMETER TEST VALUE LABEL/PARAMETER TEST VALUE
362 ALONG TK HRZ ACCEL 0.02G 014/320 MAGNETIC HEADING * 15`
332 BODY LATERAL ACCEL 0.10G 366 N-S VELOCITY 200KTS (N)
331 BODY LONGIT ACCEL 0.02G 324 PITCH ANGLE * 5`
333 BODY NORMAL ACCEL 0.10G 336 PITCH ATT RATE 10`/SEC
326 BODY PITCH RATE 10`/SEC 334 PLATFORM HEADING 22.50`
327 BODY ROLL RATE 10`/SEC 010/310 PRESENT POS LAT * N 22`30'
330 BODY YAW RATE 10`/SEC 011/311 PRESENT POS LONG * E 22`30'
363 CROSS TK HRZ ACCEL 0.02G 360 POTENTIAL VERT SPD -600FPM
321 DRIFT ANGLE * -10` (L) 325 ROLL ANGLE * 45` (R)
367 E-W VELOCITY 200KTS (E) 337 ROLL ATT RATE 10`/SEC
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